Excellent elongation and V-bendability in addition to high strength are required of a steel sheet used in the vehicle body structure of a vehicle.
It is known that a TRIP (Transformation Induced Plasticity) steel sheet containing a retained austenite phase exhibits high strength and high elongation due to the TRIP effect.
In Patent Document 1, for the purpose of further increasing the elongation of retained austenite steel, a technique of ensuring a high fraction of a retained austenite phase thereby controlling two kinds of ferrite phases (bainitic ferrite and polygonal ferrite phase) is disclosed.
In Patent Document 2, for the purpose of ensuring elongation and shape taxability, a technique of specifying the shape of an austenite phase as an aspect ratio is disclosed.
In Patent Document 3, for the purpose of further enhancing elongation, a technique of optimizing the distribution of an austenite phase is disclosed.
In addition, in Patent Documents 4 and 5, a technique of enhancing local ductility through uniformization of the structure is disclosed.